The Origin of Species

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Transcript The Origin of Species

What is a Species?
• Speciation – the origin of new species
• The biological species concept defines a species
as a population or group of populations whose
members have the potential to interbreed with
one another in nature to produce viable, fertile
offspring, but who cannot produce viable, fertile
offspring with members of other species
Eastern and
The Origin of Species
• Reproductive Isolation – biological species
concept states that each species is isolated by
factors (barriers) that prevent interbreeding
• Prezygotic Barriers – impede mating between
species or hinder fertilization of ova
– Habitat Isolation
– Behavioral Isolation
– Temporal Isolation
– Mechanical Isolation
– Gametic Isolation
Prezygotic Barriers
1. Habitat Isolation – two species living in
different habitats within the same area will
not encounter one another
• Example: garter snakes in the same
genus that live in the same area, but one
lives mainly in water and one on land; they
will most likely NOT mate.
Prezygotic Barriers
2. Behavioral Isolation – Special signals used
by a species to attract mates
• Examples: fireflies recognize signals of
their own species, frogs have calls that
only attract their own species, and birds
have songs that only attract their own
Prezygotic Barriers
3. Temporal Isolation – Species that breed
during different times of the day, season, or
• Example: Similar frog species that do not
Prezygotic Barriers
4. Mechanical Isolation – Structural differences
in genitalia or flowers
• Example: flowers whose pollen is carried
by insects to flowers of other species;
insects whose copulatory organs may not
fit together, so no sperm would be
Prezygotic Barriers
5. Gametic Isolation – male and female
gametes fail to attract each other or are
• Example: aquatic species who release
sperm and egg into the water where the
eggs are fertilized. Gamete recognition
does not occur and the sperm are unable
to fertilize the egg. Similar to the
mechanism used by flowers to recognize
sperm (pollen) of their own species.
Postzygotic Barriers
• If a sperm cell from one species does
fertilize an ovum of another species,
then postzygotic barriers usually prevent
the hybrid zygote from developing into a
viable, fertile adult
– Reduced Hybrid Viability
– Reduced Hybrid Fertility
– Reduced Breakdown
Postzygotic Barriers
1. Reduced Hybrid Viability – hybrid zygotes fail
to develop or fail to reach sexual maturity
• Example: Some species of frogs in the
genus Rana may interbreed, however the
offspring usually do not complete
development and those that do are frail.
Postzygotic Barriers
2. Reduced Hybrid Fertility – hybrid fails to
produce functional gametes
• Example: mules – a hybrid of a donkey
and a horse. Mules cannot backbreed with
either species
Postzygotic Barriers
3. Hybrid Breakdown – offspring of hybrids
have reduced viability or fertility
• Example: different cotton species can
produce fertile hybrids, but breakdown
occurs in the next generation when
offspring of the hybrids die as seeds or
grow into weak and defective plants
Modes of Speciation
Allopatric Speciation –
speciation that takes place
in populations with
geographically separate
ranges. Gene flow is
initially interrupted
because they are
separated in space
Allopatric Speciation
Likelihood of allopatric speciation increases with
small, isolated populations; Ex. Galapagos
• Conditions:
– Mountain ranges
– Land bridges
– Large lakes reduced to several smaller lakes
• The barrier’s effect depends on the ability of the
organisms to move about
• The gene pool can be changed dramatically by
genetic drift and natural selection
Allopatric Speciation
Allopatric Speciation
Antelope Squirrels
Does Speciation Always Occur
with Allopatric Conditions?
Allopatric conditions could lead to
reproductive isolation
– Evolutionary divergence of the ring species
– Adaptive radiation of the island species
– Origin of prezygotic barriers (ex. Fruit flies)
– Origin of postzygotic barriers between
distant populations of the monkey flower
• The isolated population’s gene pool evolves
by genetic drift and natural selection;
reproductive isolation evolves as a byproduct
An Example of
Radiation on
Island Chains
Modes of Speciation
Sympatric Speciation –
speciation that takes place
in geographically
overlapping populations;
gene flow is reduced due
to chromosomal changes
and nonrandom mating
Sympatric Speciation
• In plants
– Autopolyploids have more than 2
chromosome sets
– Allopolyploids are hybrids of two
different species with more than 2
chromosome sets
Sympatric Speciation
in plants
in plants
Sympatric Speciation
In animals
– Genetic factors could cause “daughter”
populations to become dependent on
resources other than the parent population
– Ex. Fig-eating wasps may prefer other fig trees
to lay their eggs. Over time the two wasp
populations may diverge into two species
– Ex. Cichlids in Lake Victoria show sexual
dimorphism and females tend to mate only with
males who have their same coloration although
they are able to mate with both morphs. Over
time the two cichlid populations may diverge
into two species
Punctuated Equilibrium Model
• Paleontologists rarely find gradual
transitions of fossil forms.
• They often observe species appearing
as new forms rather suddenly in a layer
of rock, persisting essentially
unchanged, and then disappearing from
the fossil record as suddenly as they
Punctuated Equilibrium Model
• Punctuated Equilibrium – species
diverge in spurts of relatively rapid
Long periods
of stasis
by episodes
of speciation.
From Speciation to Macroevolution
Microevolution – change over
generations in a population's allele
frequencies (by genetic drift and natural
Speciation – population’s genetic
divergence from its ancestral population
that results in reproductive isolation.
Macroevolution – Level of change
evident over the time scale of the fossil
Morphological Transformations
• The evolution of complex structures
from simpler versions (ex. eyes)
• Exaptations – structures that evolve in
one context, but become co-opted for
another function (ex. birds hollow bones
must have had some
adaptation on the
Evo-devo – interface between evolutionary
biology and the study of how organism’s
– Allometric Growth –
variations in relative
rates of growth of
various parts of the
body – giving the
body its form
Heterochrony – modification of allometric
growth. Affects evolution by altering the
rates of development of certain body parts
Example: differences in feet of salamanders
that live on the ground vs. those that live in
trees (timing of foot development)
Paedomorphosis – sexually mature stage of
a species many retain body features that
were juvenile structures
Species Selection
Species that endure the longest and
reproduce the most offspring, determine the
direction of major evolutionary trends
Evolution is never-ending, it is still going on
Evolution is a response to interactions
between organisms and their current
environments. If conditions change, an
evolutionary trend may cease or even
reverse itself.